For those interested in this topic here is a summary of our conclusions.
The really intriguing thing about Split in Mathematica 3.0 is its
apparent non-linear scaling on the Macintosh. As the result, the
fastest of our user defined versions, which scales linearly, beats the
built in function for speed for large lists. (The exact point where the
user defined verion overtakes the built-in one varies according to
processor, but occurs earliest on G3 Macs. If any one would like to
test this behaviour I can provide a notebook fro this purpose).
We have contacted technical support. They confirmed that the algorithm
contains no non-linearities, and is the same on all platforms. On other
platforms Split seems to scale linearly and easily beats all user
defined versions. We were told that Mathematica development team is
investigating this matter.
My personal opinion is that this has something to do with the compiler.
If so it would be interesting to test scaling of other built in
functions.
Andrzej Kozlowski